Origin, transport, and retention of fluvial sedimentary organic matter in South Africa’s largest freshwater wetland, Mkhuze Wetland System

Sedimentary organic matter (OM) analyses along a 130 km-long transect of the Mkhuze River from the Lebombo Mountains to its outlet into Lake St. Lucia, Africa’s most extensive estuarine system, revealed the present active trapping function of a terminal freshwater wetland. A combination of organic bulk parameters, thermal analyses, and determination of plant waxes, and their corresponding stable carbon (δC) and hydrogen (δD) isotopic signatures in surface sediments and local plant species enabled characterization and comparison of sedimentary OM in terms of stability, degradation status, sources, 5 and sinks within and among the respective sub-environments of the Mkhuze Wetland System. This approach showed that fluvial sedimentary OM originating from inland areas is mainly deposited on the floodplain and Mkhuze Swamps. In contrast to samples from upstream areas, a distinctly less degraded signature characterizes the sedimentary OM in the northern section of Lake St. Lucia. Although lake sedimentary plant waxes are similar in the observed wax distribution pattern and δC values, they exhibit considerably higher δD values. This offset in δD indicates that lakeshore vegetation dominates plant-derived 10 sedimentary OM in the lake, elucidating the effective capturing of OM and its fate in a sub-tropical coastal freshwater wetland. These findings raise important constraints for environmental studies assuming watershed-integrated signals in sedimentary archives retrieved from downstream lakes or offshore.

with about 60 % of precipitation occurring during the austral summer months (November through March) in association with cold fronts moving northward along the coast. Precipitation gradually decreases from east to west (see Figure 2, Van Heerden and Swart, 1986) from 1000 mm/year to 600 mm/year (Hutchison, 1976;Maud, 1980). Flooding is highly variable and usually associated with cutoff low pressure systems that develop during December and January, or infrequent tropical cyclones. Evapotranspiration rates are considered relatively high, ranging from 80 mm per month in winter to 190 mm per month in summer 115 (Watkeys et al., 1993).

Man-made changes
The course of the Mkhuze River has been greatly altered by human intervention, beginning in the early 1970s. A prolonged drought (1968 -1971) resulted in hypersaline conditions in Lake St. Lucia. The authorities attempted to increase the fresh water supply to the lake by excavating a canal (Mpempe Canal from near Mpempe Pan to 1 km south of Demazane Pan). Flooding 120 caused by Cyclone Domoina in 1984 resulted in severe erosion and the formation of a new stream between Tshanetshe Pan and Mpempe Pan (Taylor, 1986). Additional dredging of a channel (Tshanetshe Canal) in 1986 by a local farmer (Neal, 2001) resulted in the fact that today much of the Mkhuze River water is diverted through the Tshanetshe-Demazane Canal System (Stormanns, 1987;Neal, 2001; Barnes et al., 2002;Ellery et al., 2003). Scientific evaluation of the actions taken and their consequences for the system has been overwhelmingly negative (e.g., Alexander, 1973;Taylor, 1982b). However, Ellery et al.  Figure   1). In addition to vegetation cover, precipitation isolines (left figure) and evaporation isolines (right figure, in mm/year) are overlaid (adopted from Taylor, 1982a).
reader is referred, but also emphasizes that the alteration of the Mkhuze River flow would also likely have occurred naturally.
One aspect which is referred to the channelization processes is a change in vegetation cover. It is reported that formerly extensive stands of Cyperus papyrus within the floodplain area were reduced in extension and partly replaced by species which are tolerant to frequent inundation instead of permanently flooded conditions, such as Cyperus natalensis and Echinochloa 130 pyramidalis (Stormanns, 1987;Neal, 2001).

Sampling
Collection of samples took place during a field campaign in November/December 2018. Ten plant samples were collected. If possible, replicate plant species were sampled at various sampling sites within the system. The different species were selected based on the occurrence of large cohorts in the field or based on high reported occurrences in previous studies (Stormanns,135 1987; Neal, 2001), but not all major plant communities mentioned could successfully be sampled during the field campaign.
These include aquatic plants from the Nymphaceae family (n = 2) as well as the aquatic plant Phragmites australis (n = 2) growing on both dry and flooded soils, two species of wetland grasses, namely Vossia cupidata (n = 2) and Cynodon dactylon (n = 2), and two representatives of the Cyperaceaea family, namely Cyperus papyrus (n = 1) and Cyperus alternifolius (n = 1).
A total of 41 surface sediment samples (uppermost 10 cm) were collected along the course of the Mkhuze River and a transect 140 extending into North Lake (northern part of Lake St. Lucia, Figure 1). The collection of sediment samples was conducted under permit from Ezemvelo KZN Wildlife and iSimangaliso Wetland Park Authority.

Bulk organic matter analyses
Bulk organic analyses for determining total carbon and nitrogen content and bulk carbon isotope signatures were performed at MARUM, University of Bremen. About 10 mg of decalcified samples were wrapped in a tin capsule and analyzed with a continuous-flow elemental analyzer-isotope ratio mass spectrometer (ThermoFinnigan Flash EA 2000 coupled to a Delta V Plus IRMS). The combustion oven (filled with quartz wool, chromium oxide, and silvered cobaltous-cobaltic oxide), in which 150 C-and N-containing compounds are oxidized, was operated at 999°C. This was followed by reduction of the resulting nitrogen gases in the reduction reactor (filled with quartz wool and copper reduced granulate) operated at 680°C. Water formed was removed in a water trap (filled with magnesium perchlorate). Finally, N 2 and CO 2 were separated chromatographically (using an IRMS steel separation column for NC; length 300 cm, OD 6 mm, ID 5 mm, kept at 40°C) and transferred on-line to IRMS via a Conflow IV interface. Helium as carrier gas and oxygen as oxidation reagent were used at flow rates of 100 ml/min and  .0 V ± 0.5 V at m/z 44) was calibrated using IAEA-CH-6 international standards. Quantification of total nitrogen and organic carbon was achieved by external standard calibration using peak areas that yielded a linear 5-point calibration curve. Repeated analysis of an internal laboratory standard cross-referenced to the certified IAEA-CH-6 international standard yielded a 160 precision and accuracy of 0.2 ‰each. Sample concentration data are reported blank corrected.
Thermal analyses were performed at IFP Energies Nouvelles Lab (Rueil-Malmaison, France) using a Rock-Eval ® 6 device (Vinci Technologie). About 80 mg of the dry ground sample was pyrolyzed in an inert atmosphere (N 2 ) by heating from 200°C to 650°C at 25°C/min, then residual carbon was combusted in air from 300°C to 850°C at 20°C/min (Espitalie et al., 1985;Disnar et al., 2003). Gases released were monitored by a flame ionisation detector (FID) for hydrocarbon compounds 165 (HC), and by infrared detectors (IR) for CO and CO 2 . Total Organic Carbon (TOC in wt-%), Mineral Carbon (MinC in wt-%), Hydrogen Index (HI in mg HC/g TOC −1 ) and Oxygen Index (OI in mg CO 2 /g TOC −1 ) were calculated by integrating the amounts of HC, CO, and CO 2 produced during thermal cracking and combustion of OM ort thermal decomposition of carbonates between defined temperature limits (Behar et al., 2001;Lafargue et al., 1998). Since cracking temperature of organic compounds depends on their structural stability, the thermal status of OM was characterized by combining R-index (i.e., relative 170 contribution of most thermally stable HC pools) and I-index (i.e., ratio between thermally labile and resistant HC pools; details in Sebag et al., 2016). As derived from a mathematical construct, if the gradual decomposition of labile compounds is its main driver, OM composition can be described as a continuum from biological tissues to a mixture of organic constituents derived from OM decomposition and plotted along a linear regression line (called "Decomposition line"; Malou et al., 2020) in the I-index vs R-index diagram (called thereafter I/R diagram; Albrecht et al., 2015). However, situations with OM mixture from 175 different sources or where decomposition is so intense that it even affects the more thermally stable pools may generate a distribution diverging from the "Decomposition line". In addition, since decomposition temperature of carbonates depends on their composition, examination of CO 2 and CO thermograms enables to identify the carbonate minerals present in the mineral matrix (Pillot et al., 2014;Sebag et al., 2018).

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Analyses were performed using a FOCUS gas chromatograph coupled to a flame ionization detector (GC-FID). The GC oven hosted a Restek Rxi-5ms capillary column (30 m x 250 µm x 0.25 µm). The inlet temperature was set to 260°C and splitless injection mode was used. The GC oven was set at 60°C, held for two minutes, increased to 150°C with a heating rate of 20°C/min, subsequently followed by an increase of 4°C/min to the final temperature of 320°C, which was held for 11 minutes.
Quantification of the long-chain n-alkanes was performed by external standard calibration using the peak areas. The external 185 standard used for this purpose contains n-alkanes (C 19 to C 34 ) at a concentration of 10 ng/µL each and was measured repeatedly after all six samples, achieving a relative standard deviation of %RSD < 09.2 %. A blank sample containing only the internal standard (ISTD) and a double blank sample not containing the ISTD were also measured to ensure that no contamination occurred during the sample preparation and measurement.
Compound-specific δ 13 C values of the long-chain n-alkanes were determined using a TRACE GC Ultra equipped with an Ag- Standard deviations of replicate analyses of all n-alkanes analyzed (C 23 to C 35 ) were less than 3 ‰ (C 25 : 1.0 ‰ ± 0.59 ‰,
The H3 factor was repeatedly measured and gave a value of 4.8 ± 0.1 over the whole measurement series.

Distributional parameters of n-alkanes
The carbon preference index (CPI) and the average chain length (ACL) were adapted from Cooper and Bray (1963) and Poynter and Eglinton (1990), respectively. The following calculations were made: carbon preference index: CP I = 0.5 *  Displayed are the median and the median absolute deviation of analyzed parameters.

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All statistical analyses were performed using R software (R version 4.0.3 and RStudio version 1.4.1103). To test whether statistically significant differences occurred between the sub-environments, the Kruskal-Wallis test was performed using the stat_cor_mean() function of the "ggpubr" package (version 0.4.0). When the p value indicated that differences (p<0.05) between sub-nvironments were evident, the test was supplemented with a pairwise Wilcoxon rank sum test (base package "stats") to determine which sub-environments had significant differences between them. The Benjamini and Hochberg "BH" method 225 was used to adjust p-values. Correlation coefficients were calculated by using the stat_cor function with the default Pearson method of the "ggpubr" package (version 0.4.0). Box-and-whisker plots were generated using the "ggplot2" system (version 2.3.3.3) of the "tidyverse" package (version 1.3.0) and "ggpubr" (version 0.4.0) and the geom_boxplot() function implemented here. Here, the median of the respective data is shown as a solid line, the box of the boxplot ranges from the lower to the upper hinges corresponding to the first (25th percentile) and third (75th percentile) quartiles. Here, the median of the respective data Subsequently, the actual PCA calculations were performed by using the prcomp() function (base package "stats").

Chemical parameters of bulk organic matter
Total organic carbon (TOC), as a measure of organic matter (OM) content, increased in surface sediments from the upper reach to the swamp sub-environment, where samples contained the highest amounts of TOC ranging from 0.9 % to 8.1 % (Table 1).

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Higher variance is observed for the upper reach, floodplain, and swamp sub-environment, compared to very narrow ranges of values in the delta (1.4 % -1.8 %) and lake sub-environment (1.4 % -1.7 %). Samples from the floodplain and swamp sub-environments show significantly higher C/N ratios compared to the other sub-environments (p < 0.05, Table 1).
To estimate the total amount of plant wax input to the sub-environments, the sum of the concentrations of all long-chain oddnumbered n-alkanes was considered (Table 1). When normalized to sample mass, delta and lake samples contained significantly 245 lower amounts of plant wax-derived lipids compared to swamp samples (p < 0.005). However, when normalized to organic carbon, no significant differences were detected between any of the sub-environments investigated (p > 0.05).
Bulk OM δ 13 C shows more depleted values in the samples from the upper reach and the swamp sub-environments than in the other sub-environments, but statistical evidence is present only for the distinction between the swamp sub-environment and the delta/lake sub-environment (p < 0.05).  Note that the y-axis of A and B is broken.

Distribution patterns and stable carbon isotopic composition of n-alkanes 270
All surface sediment samples analyzed contained long-chain, odd-numbered n-alkanes (C 23 to C 35 ). The carbon preference index (CPI) has average values of 6.7 ± 1.5 for all surface sediments, confirming that n-alkanes originated from plant waxes due to the characteristic odd-over-even dominance. The CPI of the collected plant samples was 9.8 ± 5.5. The average chain length (ACL) for surface sediment samples was 30.2 ± 0.6, reflecting the high proportion of longer-chain n-alkanes, and 29.1 ± 1.7 for plant samples. Both parameters showed no statistically significant difference when comparing the sub-environments. and C 33 (12.8 %). It is striking that the sum of the relative concentrations of the long-chain, odd-numbered n-alkanes is just over 70 %, the remaining is accounted for by the respective even-numbered n-alkanes, of which C 32 (15.3 %) has the largest share.
Wetland grasses, such as C 4 (δ 13 C C31 = -18.66 ‰ ± 0.21 ‰) plant V. cuspidata (hippo grass, Figure 8 E) exhibits co-290 dominant concentrations of the n-alkanes C 27 (27.9 % ± 0.9 %) and C 29 (26.8 % ± 2.3 %) homologues going along with the occurrence of the very-long-chain n-alkanes C 31 (11.7 % ± 1.6 %), C 33 (8.9 % ± 2.5 %), and C 35 (9.4 % ± 1.5 %). C. dactylon (bermuda grass, Figure 8 F) another C 4 (δ 13 C C31 = -22.10 ‰ ± 0.06 ‰) wetland grass is clearly determined by the low dispersion around the dominant very-long-chain n-alkane C 33 dictating the appearance of the distribution with a 56.7 % ± 8.8 % share (C 31 = 23.5 % ± 11.8 % and C 35 = 8.1 % ± 6.5 %).   Figure 9 shows the relative concentrations of n-alkanes and their stable isotopic signatures for carbon and hydrogen in surface sediments within each sub-environment of the Mkhuze wetland. For simplicity, not all individual n-alkanes are displayed, but a reduced representation is shown. For reduction, the individual n-alkanes are grouped so that only the parameters of their representatives are shown to illustrate the exemplary trends. This grouping was made on the basis of visual criteria (trends 300 across the wetland system) and verified by statistical means. The results of a principal component analysis (not shown) provide information about variables that contain redundant information. For further validation, the coefficients of the linear correlation between the individual variables were used. All methods show that the following grouping is justified: (i) the parameters shown for the C 25 n-alkane also reflect trends in the C 23 n-alkane (contribution: R 2 = 0.72, p < 0.001, δ 13 C: R 2 = 0.91, p < 0.001);

Plants n-alkane distribution patterns as indicators of variable hydrological conditions
Differences in n-alkane distribution patterns between plant species have previously been observed in numerous studies (Ficken et al., 2000;Carr et al., 2014;Badewien et al., 2015;Liu et al., 2018). We are aware that the use of n-alkane distribution patterns to distinguish plant species and chemotaxonomic fingerprinting approaches are more controversial when transferring findings from one area to another, as it has been shown that variations can also occur within specific species and even between plant 335 parts of the same species (Bush and McInerney, 2013). Because all investigated plants, however, are from the same system and, when possible, multiple plants of the same species were sampled in different sub-environments of the wetland system, we believe that influences such as variability due to different climatic growing conditions and intra-species variability is small.
Aquatic plant species such as the floating Nymphaceae spp. (common water lily) and the emergent wetland sedge P. australis The riparian zone along the Mkhuze River is dominated by typical woody plants of riparian forests, such as A. xanthophloea (fever tree) and F. sycomorus (Sycamore fig) (Neal, 2001). While no representative of these species was sampled in this study, woody plants, such as trees, are generally well studied (Vogts et al., 2009). Tropical C 3 trees are typically characterized by high 345 C 29 and C 31 n-alkane contributions (in sum > 75 %), while the adjacent homologues show concentration mainly below 5 %.
However, the distribution patterns of n-alkanes show that this criterion can also be fulfilled by plants that cannot be described as woody, so that the interpretation of these specific alkanes should always be confirmed through further information, e.g., dominant vegetation form, hydrological conditions. produce C 31 n-alkanes, a finding consistent with the limited data available (Collister et al., 1994). Their natural occurrence is restricted to permanently flooded soils.
C 4 grasses such as V. cuspidata (hippopotamus grass) or C. dactylon (Bermuda grass) (Figure 8 E and F), like many (sub)tropical grasses, are characterized by the production of the very long-chain n-alkanes (C 33 and C 35 ) (Rommerskirchen et al., 2006;Vogts et al., 2012). These wetland grasses are mostly tolerant of intermittent soil flooding and other disturbances 355 (Holm et al., 1977), so they generally occur widely.

Spatial comparison of organic matter characteristics in surface sediments
Clear differences between the individual sub-environments of the Mkhuze Wetland System become evident by comparing the organic matter characteristics with respect to stability, degree of degradation, primary contributing vegetation and its hydrological growth conditions. That these differences sometimes appear small in absolute values is attributable to the system itself. in a process-oriented manner (Reuter et al., 2020;Reddy and DeLaune, 2008, and references therein). Fresh OM, such as plant debris often shows very high C/N ratios, approaching a value of 10 during decomposition, which is due to dilution with bacterial biomass, which decomposes the organic matter and uses it to build up further biomass and produce energy through mineralization, having a C/N ratio of 10.
The concentration of plant wax-derived n-alkanes, biomarkers that are characterized by their high refractory properties, also 375 elucidates information on stability and quality of the organic matter. When comparing their concentration normalized to dry weight and organic carbon, respectively, conclusions about whether they either reflect remnants of degraded OM leading to their relative enrichment or input of fresh OM can be drawn.
Plant wax n-alkane distributions and associated stable isotopic signatures (see subsection 4.1 as plant characteristics are used as a system's vegetation in-situ calibration to assess the dominant source vegetation.

Upper reach of the Mkhuze River
Bulk organic matter in surface sediments of the upper reach is generally low in concentration (see Table 1 and Figure 5 A) and shows geochemical and thermal characteristics of degraded OM (HI ∼ = 120 mg/g TOC, C/N = 11.8 ± 1.0; Figure 6 A and OM in the surface sediments of the upper reach therefore reflect allochthonous contributions from the hinterland, since the 390 Mkhuze River is primarily lined with riparian forests (Neal, 2001). These trees do not tolerate flooding or water saturated soil conditions so that they are found on the elevated channel levees. This interpretation of the hinterland as origin is further supported by the stable hydrogen isotopes of the respective n-alkanes, particularly evident in C 29 (Figure 9 F). The relatively enriched δD values suggest that the plants producing them were exposed to relatively dry conditions during their growth phase.
These dry conditions are met to a large extent in the hinterland, considering that the precipitation gradient across the Mkhuze

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Wetland System is nearly halved from 1000 mm/year in the east near the coast to 600 mm/year at the Lebombo Mountains (Maud, 1980, Figure 2).

Floodplain
Bulk organic matter in surface sediments of the floodplain is characterized by high variability in both quantity (TOC, Table   1 and Figure 5 A) and quality (HI, R-and I-indexes, Figure 6 A-C). It splits into three groups, the first one corresponds to 400 samples poor in OM (TOC < 1.5 %) and in hydrocarbon compounds (HI < 80 mg HC/gTOC). A second group corresponds to samples rich in OM (TOC > 5 %) and hydrocarbon compounds (HI > 150 mg HC/gTOC). The third group presents an intermediate situation (TOC≈2-3 %, HI ≈ 100 mg HC/g TOC). An explanation could consider that this gradual evolution corresponds to a more or less advanced degradation of the sedimentary OM. However, the I-index does not support this interpretation, since the samples with the lowest HI present the least advanced degree of decomposition of thermally labile compounds (I > 0.1), 405 and conversely. In addition, floodplain samples show a wide dispersion in the I/R diagram (Figure 7) which excludes a gradual decomposition. By analogy with previous work on soils, such nonlinear signatures would rather be associated with OM mixtures of varying quality and origin, which is consistent with such a heterogeneous depositional environment.
Mixing of OM is further corroborated by the plant wax concentration per dry weight and organic carbon (Table 1 and Figure   5 C-D) showing great variances and therefore that n-alkanes are partly relatively enriched during degradation of OM but also 410 inputs of fresh organic material which is in line with elevated C/N ratios (Figure 5 B).
High variability is also evident in the relative contributions of the n-alkanes (Figure 9 A, D, G) and the corresponding isotopic signatures (Figure 9 B the parameters. However, it is noteworthy, despite this predominant scatter, that the strongest input of C 4 vegetation is clearly observed in floodplain sedimentary organic matter (Figure 9 B, E, H). This cannot be explained solely by the occurrence of C 4 grasses such as C. dactylon or Echinochloa pyramidalis, which are recognized as important floodplain vegetation communities (Neal, 2001), since the "grassy" n-alkane input is not significantly higher than in the downstream swamp (Figure 9 G). In addition, Neal (2001) describes the floodplain is increasingly used for growing crops, such as the C 4 crops sugarcane or corn 420 (Figure 3), which could explain the particularly strong C 4 signal.

Swamp
The bulk organic matter in surface sediments of the swamp sub-environment is similarly characterized by high variability in both quantity (TOC, Table 1  (delta, lake) sub-environments, and therefore reflects a transitional sub-environment.
Furthermore, the organic matter is characterized by comparatively high C/N ratios, i.e., corroborating addition of in-situ produced organic matter to the samples. This is also seen in the summed concentration of n-alkanes normalized to dry weight being very high, while normalized to organic carbon being rather low what is attributed to fresh organic matter inputs.

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A similar pattern like for the bulk organic matter can also be observed in the plant wax data. While the n-alkane distribution closely resembles that of the floodplain (Figure 9 A, D, G), the corresponding carbon isotope signatures (Figure 9 B

Delta and lake
The organic matter of the lake and delta samples shows striking differences when compared with that of the upstream sub-445 environments of the Mkhuze Wetland System.
The bulk organic matter in surface sediments shows a homogeneous signature with lowest contents in OM (TOC < 1.6 %, Table 1 and Figure 5 A) and hydrocarbon compounds (HI < 80 mg HC/g TOC, Figure 6 A) of all sub-environments, and a high degree of preservation of thermally labile fractions (I > 0.3, partly in the upper limit of the "decomposition line"). These results differ drastically from the OM results of the upstream sub-environments, but they do not reflect aquatic autochthonous contri-450 butions (as indicated by the low HI). Although the sources of this OM are probably terrestrial, it is not a detrital (allochthonous) OM, reworked from the catchment area, but rather a proximal (para-autochtonous) contribution.
The concentration of n-alkanes normalized to dry weight is comparatively low, but normalized to organic carbon no differences are observable, signifying the input of fresh material.
In contrast to bulk OM, the contribution of aquatic, woody, and grassy n-alkanes (Figure 9 A, D, G) and the corresponding

Transport of sedimentary organic matter in the Mkhuze Wetland System
Characterization of organic matter in surface soils and sediments in terms of its stability, degree of decomposition and source vegetation, as well as their hydrological conditions in the sub-environments of the Mkhuze Wetland System, allows us to evaluate transport pathways. Plant-wax lipids are hydrophobic and associated with the mineral component of sediments (Hedges and Keil, 1995;Keil et al., 1997;Wiesenberg et al., 2010). Thus, the transport and identification of sources and sinks is not Cores retrieved from Lake St. Lucia (Benallack et al., 2016) and the Mkhuze River bayhead delta (Humphries et al., 2020)  The high concentration of plant waxes in the upper reach of the Mkhuze River is due to low flow conditions during the sampling campaign. During the spring season, the river typically has low or even no flow (McCarthy and Hancox, 2000), resulting in the deposition of suspended sediment in the riverbed. The upper reach samples collected in this study thus likely reflect the "undiluted" hinterland signal (highly degraded, stable, C 3 woody). During periods of high flow, transported fluvial OM is deposited along the river channel and on the floodplain by bank overtopping (Neal, 2001;Ellery et al., 2011). The even

Local ecological implications
The identification of the Mkhuze Swamps as the ultimate sink for suspended OM from the Mkhuze River confirms previous studies that the Mkhuze Wetland System, specifically the floodplain and swamp, acts as an efficient filter upstream of Lake St.
Lucia (Taylor, 1982b;Stormanns, 1987). The current active filtering and trapping function of high sediment loads, including organic sedimentary organic load from the Mkhuze River, may prevent the otherwise rapid siltation of Lake St. Lucia.

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OM in the surface sediments of Lake St. Lucia originates primarily from lakeshore vegetation, as indicated by the lake transect data shown in comparison with upstream areas of the system. However, some studies (Taylor, 1982b) assume that the Mkhuze Swamps, in their function as freshwater reservoirs ("sponges") (Alexander, 1973), are also responsible for input of OM, serving as a potential energy source in Lake St. Lucia. Our data on particulate organic matter contradict this assumption showing that sedimentary particulate OM is presently not transported from the hinterland nor exported directly from the swamp.

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In contrast, OM export from wetlands occurs primarily through the export of dissolved organic matter (DOC, Cole et al., 2007), which accounts for about 90 % of total OM (Reddy and DeLaune, 2008). In saline waters, like Lake St. Lucia, DOM is likely to flocculate (Ardón et al., 2016). Assuming that OM is exported in dissolved form from the Mkhuze Swamps, it should thus be detectable in the lake transect surface samples, but this was not observed in this study. In part, this could be due to the fact that a significant porportion of DOC may be removed by sorption onto precipitating oxides when sediments contain 510 substantial amounts of aluminum and iron metal oxides (McKnight et al., 1992), as is the case in upstream sub-environments.
With the employed methods we cannot confirm neither any particulate OC nor DOC export from the Mkhuze Wetland System into Lake St. Lucia.
The present study is the first examining sedimentary OM transport within the Mkhuze Wetland System, revealing that OM is indeed transported even to the Mkhuze Swampsand not being just deposited near the river channel and on the floodplain. Three 515 processes might play a role for transport of material into the swamps: (i) the ongoing eastward progression of the floodplain (McCarthy and Hancox, 2000), (ii) transport during severe flood events caused by cyclones and cutoff lows , or (iii) that channelization has had an impact on the transport efficiency. The transport path of the Mkhuze River has been dramatically shortened by channelization (see section 2.1.3). As a result, most of the Mkhuze River water now flows through the Tshanetshe-Demazane Canal System (Stormanns, 1987;Neal, 2001;Barnes et al., 2002;Ellery et al., 2003), which may have altered 520 sediment transport. A shift in the area of deposition of material transported by the Mkhuze River is likely to affect the local vegetation distribution, i.e. causing a shift in the ecological zones by altered substrate conditions. In general, however, the Mkhuze Wetland System overall appears to exhibit high resilience against natural and/or anthropogenic induced changes. The severe drought of 2016, which led to the drying of large parts of Lake St. Lucia, does not seem to have had any lasting impact on the filtering function of the swamps. Likewise, the establishment of the canal system also does not appear to have caused 525 lasting damage to the filtering function of the Mkhuze Swamps.

Fate of sedimentary organic matter in wetlands
In comparison with humid region (tropical and temperate) wetlands, the Mkhuze Wetland System exhibits distinctive characteristics that reflect the low ratio between precipitation and potential evapotranspiration characterizing the region (Figure 2).
High evaporative demand and transmission losses from the river to the surrounding floodplain, result in marked declines in both 530 channel width and depth downstream (Humphries et al., 2010). Downstream decreases in discharge and stream power result in a gradual decline in ability of the Mkhuze River to transport particulate material, ultimately terminating in the Mkhuze swamps.
Although typically unusual, such downstream changes appear to be distinctive features of wetlands found in sub-humid and semi-arid regions of the world (Tooth and McCarthy, 2007), and is likely an important reason why the Mkhuze Wetland System acts as such an efficient trap for organic material.

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Most large tropical and temperate river systems are associated with wetlands (Wetzel, 2001;Ward et al., 2017) along their river courses. For example, studies conducted on the Cuvette Congolaise (Runge, 2007), a large wetland system traversed by the Congo River, indicate that fluvially transported particulate OM signals from upstream sources are seasonally overprinted by storage and release of particulates in and from the wetlands (Hemingway et al., 2016(Hemingway et al., , 2017 (Hemingway et al., 2017). Wetlands may thus switch from a trapping function to export of carbon depending on hydrological conditions and seasonal climatic changes.
This differential trapping and export functions of wetlands need to be considered when reconstructing climatic changes based on sedimentary archives recovered from terminal lakes and offshore archives. Depending on the activity and efficiency 545 of wetlands, material from more upstream areas will effectively masked by wetlands depending on their hydrologic state and can even be overprinted be wetland export of OM or OM input from downstream areas. Such a process has been suggested for the transport of OM in the Amazon River system, where Andean material is effectively overprinted by lowland sources from rainforests, floodplains and wetlands (Quay et al., 1992;Blair et al., 2004). In such cases, reconstructing environmental changes in the integrated watershed using offshore archives may thus not be possible. Wetland systems with an active trapping 550 function effectively change the transported OM, so that signals detected in offshore archives instead reflect specific sections of the river catchment. Other sediment related proxies may also be affected by wetland trapping, so such geomorphological settings could have a much stronger influence than is often assumed. Combining environmental analyses with specific markers released from wetlands, on the other hand, allows an assessment of the hydrologic changes that lead to inefficient OM trapping and degradation of wetlands (e.g., Schefuß et al., 2016).

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In addition, carbon sequestration and the ability of wetlands to act as carbon sinks are considered to play an important role in the global carbon budget. There is concern that global warming may alter the hydrological balance of wetlands, releasing significant amounts of carbon to the atmosphere through direct oxidation processes or to adjacent water bodies through erosion of wetland soils, as has been observed for the Cuvette Congolaise and elsewhere (Hemingway et al., 2017). In certain cases, extreme weather events, which are expected to become more frequent as the global climate changes, have also been shown to 560 promote the release of DOC from wetlands (Rudolph et al., 2020). It is likely that particulate material would also be exported by excessive flooding, as is similarly observed by increased flushing of terrestrial carbon into river systems (Bianchi et al., 2013). Thus, the trapping function of wetlands, overridden by overloading, would just as likely contribute to increased carbon dioxide emissions to the atmosphere through turnover of exported OM in adjacent waters.

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We present a spatial assessment of TOC concentrations, OM composition (δ 13 C, C/N, HI, R-index, I-index), n-alkane distributions, and their respective compound-specific stable carbon (δ 13 C n−alkane ) and hydrogen (δD n−alkane ) isotope compositions along an approx. 130 km-long transect of the Mkhuze River and plant wax data from locally dominant plant species to constrain the origin and transport pathways of OM through and within the sub-environments of the Mkhuze Wetland System, South Africa. Our results indicate that degraded OM originating from the hinterland is deposited primarily on the floodplain of much lower concentrations and much less degradation due to proximal terrestrial inputs rather than aquatic contributions. Plant 580 wax data confirms these findings, pointing to lake shoreline vegetation as the main source.
This study shows that traversed or terminal wetlands under certain conditions, such as low flow, in this case a result of climatic factors, i.e., evaporation exceeds precipitation, can capture OM so efficiently that transport from upstream areas does not occur and downstream OM originates almost exclusively from the immediate vicinity.
We emphasize that such wetlands, as geomorphological features within river systems, can impact environmental studies 585 based on terminal sediments, thereby assuming watershed-integrated information. In addition, disturbances, e.g., by extreme weather events, which are assumed to become more frequent under global climate change, are likely to affect the trapping function of wetlands and thus increase export of previously stored OM, leading to an increase in carbon emissions through turnover of exported OM in adjacent waterbodies.
Code and data availability. The research data has been submitted to Pangaea, but a DOI is not yet assigned.
-Center for Marine Environmental Sciences. We thank them for their support in chemical analyses and the student assistant Abdullah Saeed Khan to assist in bulk organic matter analyses. We thank Prof.